1. Field of the Invention
The present invention relates to novel aqueous dispersions of a siliconate-based silicone, which novel dispersions are crosslinkable into elastomeric state by elimination of water therefrom.
2. Description of the Prior Art
U.S. Pat. No. 2,891,920 describes a process for emulsion polymerization of polydiorganosiloxanes utilizing an acidic or basic catalyst in the presence of anionic, cationic or nonionic surface-active agents. The '920 patent relates that the emulsions obtained are stable in storage and, after addition of fillers, can be used to formulate paints for providing continuous coatings by elimination of water therefrom.
U.S. Pat. No. 3,294,725 describes, in particular, the use of dodecylbenzenesulfonic acid to polymerize polydiorganosiloxanes in emulsion. This '725 patent observes that to obtain stable emulsions it is desirable to adjust the pH of such emulsions to a value of approximately 7. It also relates that an elastomeric coating can be obtained from these neutralized emulsions, to which colloidal silica and a polyalkoxysilane have been added.
U.S. Pat. No. 3,360,491 is similar to the '725 patent, except that dodecylbenzenesulfonic acid is replaced with lauryl hydrogen sulfate.
U.S. Pat. No. 3,697,469 describes a particular process for emulsion polymerization of polydiorganosiloxanes and indicates the possibility of adding colloidal silica and a tin salt to the emulsion in order to provide an elastomeric coating by water evaporation.
French Patent FR-A-2,110,358 describes a silicone emulsion having a pH of between 6.5 and 9, crosslinking to an electrically conductive elastomer after evaporation of water and incorporating carbon black. The emulsion, additionally containing a tin salt and a polyalkoxysilane, is not stable in storage and must be stored in two separate packages (two-component emulsion).
U.S. Patents Nos. 4,221,688 and 4,244,849, and French Patent FR-A-2,463,163, describe storage-stable silicone emulsions comprising:
(i) an anionically stabilized emulsion of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane polymer;
(ii) a siliceous filler;
(iii) a tin salt; and
(iv) optionally, a nonreinforcing filler.
The siliceous filler may be a colloidal silica (U.S. Pat. No. 4,221,688), sodium silicate (U.S. Pat. No. 4,244,849) or an amorphous silica powder (FR-A-2,463,163).
In comparison with the known aqueous emulsions (dispersions) of the prior art, these three patents offer, on the one hand, that to obtain a storage-stable single-component emulsion, the emulsion must be stored at an alkaline pH higher than 8.5 or 9, preferably higher than 10 and, on the other hand, a tin salt must be incorporated in the emulsion to shorten to a few days the emulsion ripening or aging stage required to provide a crosslinkable dispersion.
U.S. Pat. No. 3,355,406 describes a silicone latex of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane, preferably prepared by emulsion polymerization, and of a sesquisiloxane resin including RSiO.sub.1.5 recurring units (R=hydrocarbon residue). The latex may additionally comprise a metal curing catalyst and an alkyltrialkoxysilane.
In U.S. Pat. No. 4,554,187, the silicone resin used in combination with the .alpha.,.omega.-(dihydroxy)polydiorganosiloxane is a reactive resin of low molecular weight, containing alkoxy or acyloxy groups.
In patent application Ser. No. EP-A-266,729, the silicone resin used in combination with the .alpha.,.omega.-(dihydroxy)polydiorganosiloxane and with the curing catalyst is a siliconate.
A silicone resin comprising up to 10% by weight of hydroxyl groups may be used in combination with this siliconate.
Catalyst-free silicone latices are described in Patents EP-A-166,396 and EP-A-169,386, as well as in EP-A-277,740, but they do not comprise any silicone resins.
Examination of the prior art indicates that most of the aqueous silicone dispersions comprise a curing catalyst which is a metal salt, generally an organotin salt.
However, the presence of such metal salt is not without disadvantages. It may be, in fact, the source of an inadequate storage stability. Furthermore, for coating medications and for coating articles destined to be in contact with foodstuffs and beverages, such as cork stoppers, serious need continues to exist in this art for organopolysiloxane compositions capable of crosslinking into a silicone elastomer without the requirement for a metal curing catalyst.